1. Field of the Invention
The present invention relates to a drive circuit for a current sense IGBT (Insulated Gate Bipolar Transistor) that can detect the current through the IGBT itself, and particularly to a drive circuit that can be applied to power conversion apparatuses such as inverters to protect the IGBT from an overcurrent caused by faults like accidental shorts.
2. Description of the Prior Art
An IGBT is a new device having the advantages of both a bipolar transistor and a power MOSFET. High withstanding voltages and large current capacities can be easily achieved as in a bipolar transistor, and high switching speed and easy driving can be attained, as in a MOSFET
FIG. 1 is a circuit diagram showing the equivalent circuit of an N-channel IGBT. Incidentally, throughout the following drawings, like reference numerals designate like or corresponding parts.
In FIG. 1, the IGBT is composed of an N-channel MOSFET 1, an NPN transistor 2, a PNP transistor 3, and a resistor 4 connecting the base and emitter of the transistor 2. The drain-source of the MOSFET 1 and the emitter-collector of the transistor 2 are connected in parallel, and the transistors 2 and 3 can be represented as constituting a thyristor circuit.
Recently, IGBT devices incorporating a current detecting function have been developed to further improve the performance thereof. A current sense IGBT includes a current detecting chip incorporated in the IGBT chip. The current flowing through the current detecting chip is detected and a detection signal is generated.
FIGS. 2A and 2B are diagrams showing the symbol and the equivalent circuit of a current sense IGBT, respectively. As shown in FIG. 2B, the current sense IGBT Q1 is composed of a main IGBT Qm and a current detecting IGBT Qs which are connected in parallel. The current Ic of the current sense IGBT Q1 is divided between the main IGBT Qm and the current detecting IGBT Qs. The current flowing through the current detecting IGBT Qs is determined by the ratio of the chip area the current detecting IGBT Qs to the entire chip area of the IGBT Q1. Accordingly, the current flowing through the main IGBT Qm can be calculated from the ratio.
When an IGBT is used in a power conversion apparatus such as an inverter, it is important to protect the IGBT from damage even if an overcurrent fault takes place during the operation of the inverter. A short-circuit fault is a well known overcurrent fault that may cause device damage.
FIG. 3 is a circuit diagram simulating a short-circuit fault, and FIGS. 4A and 4B are diagrams showing the voltage V.sub.CE and the current I.sub.c of the IGBT Q during the short-circuit fault, respectively. As shown in FIGS. 4A and 4B, a direct current power supply voltage Ed is applied to the IGBT Q during the short-circuit, and the current I.sub.c flowing through the IGBT Q increases to more than five to six times the rated direct current I.sub.CR of the IGBT Q. Accordingly, a large amount of power is applied to the IGBT Q during the short-circuit period. Consequently, to protect the IGBT Q during the short circuit, it is necessary to turn off the IGBT Q in a time period (about 10 .mu.m) the device can endure. Thus, using the current sense function for overcurrent protection is an effective method for protecting the IGBT Q.
FIG. 5 is a circuit diagram showing a conventional gate drive circuit for a current sense IGBT with an overcurrent protection function. In this figure, reference character Q1 designates a current sense IGBT as a main switching device, PH1 denotes a photocoupler for signal isolation, V1 denotes a voltage source for supplying an on-gate voltage, and V2 designates a voltage source for supplying an off-gate voltage.
The normal operation of the gate drive circuit of FIG. 5 will now be described. When the photocoupler PH1 is turned on, a transistor T1 turns off, thereby turning a transistor T2 on and transistor T3 off. Thus, the on-gate voltage V1 is applied across the gate and emitter of the IGBT Q1 through a gate resistor R.sub.G. On the other hand, when the photocoupler PH1 is turned off, the transistor T1 turns on, thereby turning the transistor T2 off and the transistor T3 on. Thus, the off-gate voltage V2 is applied across the gate and emitter of the IGBT Q1 through the gate resistor R.sub.G.
The overcurrent protection operation of the gate drive circuit of FIG. 5 will now be described. While the IGBT Q1 is on, a detection current I.sub.CM of the IGBT Q1 is applied to a fault discrimination operational circuit 100. The operational circuit 100 compares a reference value I.sub.OC * with the detection current I.sub.CM, and judges that an overcurrent fault is taking place when the detection current I.sub.CM is greater than the reference value I.sub.OC *. The operational portion 100 produces a signal for turning on a transistor T4. When the transistor T4 turns on, the off-gate voltage V2 is applied across the gate and emitter of the IGBT Q1, thereby interrupting the overcurrent.
The conventional gate drive circuit of FIG. 5, however, has a problem in that an overvoltage produced due to the overcurrent may damage the IGBT Q1. More specifically, when the transistor T4 is turned on, the off-gate voltage V2 is simultaneously applied to the gate of the IGBT Q1, and hence a large current reduction ratio -di/dt occurs in the IGBT Q1 during the overcurrent limitation. Thus, the sum of the voltage ldi/dt induced due to wire inductance and the DC voltage of the direct circuit is applied to the IGBT Q1, which may cause damage to the device Q1.